Conventional geothermal ground source/water source heat exchange systems typically use liquid-filled closed loops of tubing (typically approximately ¼ inch wall polyethylene tubing) buried in the ground, or submerged in a body of water, so as to either absorb heat from, or to reject heat into, the naturally occurring geothermal mass and/or water surrounding the buried or submerged liquid transport tubing. The tubing loop, which is typically filled with water and optional antifreeze and rust inhibitors, extends to the surface. A water pump circulates the naturally warmed or cooled liquid to a liquid-to-refrigerant heat exchanger.
Transfer of geothermal heat to or from the ground to the liquid in the plastic piping is a first heat exchange step. Via a second heat exchange step, a refrigerant heat pump system transfers heat to or from the liquid in the plastic pipe to a refrigerant. Finally, conventional systems may use a third heat exchange step, in which an interior air handler (comprised of finned tubing and a fan) transfers heat to or from the refrigerant to heat or cool interior air space.
Newer design geothermal DX heat exchange systems, where the refrigerant fluid transport lines are placed directly in the sub-surface ground and/or water, typically circulate a refrigerant fluid, such as R-22, R-410A, or the like, in sub-surface refrigerant lines, typically comprised of copper tubing, to transfer geothermal heat to or from the sub-surface elements via a first heat exchange step DX systems only require a second heat exchange step to transfer heat to or from the interior air space, typically by means of an interior air handler. Consequently, DX systems are generally more efficient than water-source systems because fewer heat exchange steps are requited and because no water pump energy expenditure is necessary. Further, since copper is a better heat conductor than most plastics, and since the refrigerant fluid circulating within the copper tubing of a DX system generally has a greater temperature differential with the surrounding ground than the water circulating within the plastic tubing of a water-source system, generally less excavation and drilling is required (and installation costs are typically lower) with a DX system than with a water-source system
While most in-ground/in-water DX heat exchange designs are feasible, various improvements have been developed intended to enhance overall system operational efficiencies. Several such design improvements, particularly in direct expansion/direct exchange geothermal heat pump systems, are taught in U.S. Pat. No. 5,623,986 to Wiggs; in U.S. Pat. No. 5,816,314 to Wiggs, et al; in U.S. Pat. No. 5,946,928 to Wiggs; and in U.S. Pat. No. 6,615,601 B1 to Wiggs, the disclosures of which are incorporated herein by reference. Such disclosures encompass both horizontally and vertically oriented sub-surface heat geothermal heat exchange means, using historically conventional refrigerants, such as R-22, as well as a newer design of refrigerant identified as R-410A. R-410A is an HFC azeotropic mixture of HFC-32 and HFC-125.
DX heating/cooling systems have three primary objectives. The first is to provide the greatest possible operational efficiencies, which enables the lowest possible heating/cooling operational costs as well as other advantages such as, for example, materially assisting in reducing peaking concerns for utility companies. A second objective is to operate in an environmentally safe manner by using environmentally safe components and fluids. The third objective is to operate for long periods of time absent the need for any significant maintenance/repair; thereby materially reducing servicing and replacement costs over other conventional system designs.
Historically, while DX heating/cooling systems are generally more efficient than other conventional heating/cooling systems, they present installation limitations due to the relatively large surface land areas necessary to accommodate the sub-surface heat exchange tubing. In horizontal “pit” systems, for example, a typical land area of 500 square feet per ton of system design capacity was required in first generation designs to accommodate a shallow (within 10 feet of the surface) matrix of multiple, distributed, copper heat exchange tubes. Further, in various vertically oriented first generation DX system designs, about one to two 50-100 foot (maximum) depth wells/boreholes pet ton of system design capacity are needed, with each well spaced at least about 20 feet apart, and with each well containing an individual refrigerant transport tubing loop. Such requisite surface areas effectively precluded system applications in many commercial and/or high density residential applications. An improvement over such predecessor designs was taught by Wiggs, which enabled a DX system to operate within wells/boreholes that were about 300 feet deep, thereby materially reducing the necessary land surface area requirements for a DX system. Historically, copper tubing has been used for sub-surface refrigerant transport purposes in DX system applications.